1. Field of the Invention
The present invention relates to a device for determining the after-run voltage of a direct current motor, and a corresponding method.
2. Description of Related Art
For various applications it is necessary to determine the rotational speed of direct current motors. The rotational speed may be derived from the after-run voltage when the motor is controlled in a cycled manner. Thus, in published German patent document DE 199 14 404, for example, the after-run voltage of a pump motor in the uncycled no-pulse period is used to determine the rotational speed of the pump.
In published German patent document DE 195 28 697 a method is described in which a pressure variable is determined for a feed pump, for example in a brake system, on the basis of a variable that represents a measure of the rotational speed of the feed pump.
A method is described in published German patent document DE 199 46 177 for estimating an intake pressure between a master brake cylinder and an intake valve of a wheel braking cylinder in a motor vehicle brake system. The intake pressure is estimated, taking into account an after-run voltage of a motor, operable in a cycled manner, of a pump for conveying brake fluid.
Conventionally, the after-run voltage is usually determined via averaging instantaneous values of the motor voltage in the no-pulse period. This averaging is necessary because the generator after-run voltage is permeated via rather large interference signals as the result of brush sparking. The recording of the instantaneous values is controlled, for example, via clocking by a microprocessor, it being necessary to record multiple instantaneous values to obtain a measuring signal of sufficient quality since the dispersion of the instantaneous values may be very large. The rotational speed of the direct current motor is then computed from the average value thus obtained.
It is described in published German patent document DE 103 59 224 to wait for a specifiable first time period after the no-pulse period begins before starting the determination of the after-run voltage. In this manner, fluctuations which arise immediately after the no-pulse period begins as the result of current decay are masked.